rigid foam insulation/roof application
vaughan
| Posted in Construction Techniques on
I am beginning a new house project and the ceiling application will be open/exposed rafters with cardecking sheathing over the top.With this detail I will need to place 1/2 inch ply down on cardecking (as per engineer) then it calls for rigid foam R-30 with 2×6 rafters standing inbetween at 24 inch oc., this all covered with 1/2 inch ply.I understand all this but my questions are air ventilation and vapor barriers.My location is Pacific NW if that has any bearing. Also my mind has thought about just having insul panels made up and placed on top of my” first roof”, I would think it is much easier but at what expense vs hand building of the “second roof.”Appreciate any feedback or experiences on this matter. joe
Replies
Greetings vaughan,
As a first time poster Welcome to Breaktime.
This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
You are right. SIPs is the way to go there.
You would not even be able to fit enough EPS foam in that 5,25" space to get R-30 insulation.
5.2R X 5.5" = 28.6 at best.
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Premier makes SIPs in Kent Wa. Google them and check with a planner.
Recently had this same problem, solution, ( worked out between roofer, building inspector and architect ) 1 layer of torch down roofing on top of ply. acts as vapour barrior, 5" poly-iso rigid insulation, (approx. r30) fibre glass mat, (fire protection.) 2 layers torch down ( 1 base 1 cap ) to finish. No venting required. Solid construction, no voids, no free air. Have heard it called "upside down roof" or "sandwich roof"
Thanks for that insight,so how do you place a fachia board on the face of the "inso sandwich".I cant see where the nailing of the fachia board would have backing?Must be some detail to cover this. jv
On our project, we had a parapet wall, so the "sandwich" died into it with a scupper detail for drainage. This created another problem where the wall insulation below met the roof. The inside of the parapet was not insulated, leaving only about 2" of coverage between the roof and the wall insulation. Hard to explain, but sketch a section and you get the idea. This was solved by spray foam in the roof joist bays at the perimeter and extending it far enough to capture the iso on top. If a parapet dosen't work for you, run a 2x6 on edge around the perimeter and cap it with torch down, metal flash to finish.
Rip a 2x to the thickness of the foam. dig out the foam 1-1/2" and replace the removed foam with the ripped 2x nail the 2 x in place and use it for the backer for your fachia.